The present invention relates generally to food processing equipment, and more particularly to an apparatus for continuous mixing or stir fry cooking of foods such as meats and vegetables.
The stir frying of meats and vegetables without the use of oil has become increasingly popular in recent years as the population has become more health conscious. Frying adds a unique taste to the food product while the use of no cooking oil (or at least very little) minimizes the amount of fat in the diet. By quickly frying vegetable products on a very high temperature heated surface (e.g., 425xc2x0 F. to 600xc2x0 F.) the enzymes are killed which increases shelf life, while the vegetable product maintains a crisp, fresh texture. This frying process has been used for centuries by the Asian population and in recent years has become very popular in the Western World.
The current methods of stir frying in the industrial food industry are all batch processes. A certain volume of food product is loaded into a frying vessel and stirred for a predetermined time period and then removed, and as quickly as possible thereafter the next batch is loaded. The problems associated with this batch method of stir frying are as follows:
1. Very little food product can be fried at one time. If the layer of product on the heated surface is more than 2 or 3 inches deep, the portion of the product being heated on the bottom layer gives off steam as the moisture in the product fries. This steam rises through the product above, thereby steam cooking it. If the product layers are deeper than 2-3 inches the portion of the product in the upper layers is steamed and not fried, and therefore becomes limp and soggy. For this reason the food product must be spread out in a relatively thin layer over a very large area to fry properly. This dramatically complicates the mechanical stirring of the product.
2. When batch cooking, approximately 50% of the processing time is either spent loading or unloading the product from the frying vessel. Given the small batch sizes required for proper stir frying as described above, the production rate possible in a traditional industrial fryer is very small resulting in very poor economies.
3. During the time when there is no food product in the batch frier (i.e., between the unloading and loading steps) the heated surfaces tend to over fry or burn the small amount of product that always remains after unloading. This small amount of burned product taints the next batch with off flavors.
These batch frying problems are dramatically reduced if the food product is continuously fed into a fryer and is continuously discharged. Continuous fryers have been developed that are designed similar to a screw conveyor with mixing bars or scrapes attached to the conveyor flights. However, the problem with known continuous fryers is that in order to obtain an adequate frying time (e.g., 10-15 minutes) the cooker must either be very long, or the screw must move very slowly, or both. If the screw moves very slowly, the product is not mixed adequately and does not fry evenly. If the machine is very long (e.g., 26-28 feet), the machine is very expensive and very difficult to design to avoid warping due to the high temperatures (if the machine warps the screws will rub the sides of the trough).
Even if the problems listed above are solved, the screw rotating in one direction conveys the product off the center line of the machine, piling it up on one side so that the heat exchange surface is not evenly covered. The result is that continuous screw fryers are not efficient.
The continuous mixing of food products (without frying) is also becoming increasingly important as processors look for ways to increase productivity and decrease manual labor involvement. Combining the conveying of food products with the mixing of these products is difficult for the same reasons as stated above, since the speed of conveying is usually quite slow and therefore the rotation of the conveyor auger is too slow to properly mix the products.
The continuous mixer or stir fry cooker apparatus of this invention provides a single agitator mechanism made by attaching conveying/agitating structures such as paddles or solid, continuous spiral flights to a shaft which is driven from one end about its longitudinal axis. The agitator is mounted in a trough with bearings holding the shaft at each end in such a manner as to allow the agitator to rotate freely in either direction. The shaft is driven by a reversible drive motor. Mounted on the paddles or spiral flights are lifting bars (in the case of a continuous mixer) or scrapers (in the case of a continuous fryer) which scrape the sides of the trough as well as mix or toss the food product as the agitator rotates.
The invention uses an automatic reversing system to cycle the agitator of the continuous mixer or cooker back and forth by reversing the shaft on a preprogrammed basis. The control system of the agitator is preferably programmable to allow setting the time the agitator rotates in one direction (e.g., conveying the food product from the inlet of the machine toward the outlet) and then reversing the agitator for a timed period of rotation in the opposite direction. By rotating the agitator for a period of time in the direction that conveys the product from the inlet toward the outlet, and then rotating it in the opposite direction for a shorter period of time, the product is gradually, continuously conveyed toward the outlet.
By repeating the forward, reverse cycle over and over with a fast RPM the product is vigorously stirred or tossed back and forth, assuring thorough mixing (in the case of a continuous mixer) and frequent contact of the product pieces with the heat exchange surface to maximize the frying of the product (in the case of a continuous fryer), at the same time the product is conveyed slowly toward the discharge end of the machine.